ADDITIONAL NOTES. NOTE I ... METEORS. There are four strata of the atmosphere, and four kinds of meteors. 1. Lightning is electric, exists in visible clouds, its short course, and red light. 2. Shooting stars exist in invisible vapour, without sound, white light, have no luminous trains. 3. Twilight; fire-balls move thirty miles in a second, and are about sixty miles high, have luminous trains, occasioned by an electric spark passing between the aerial and inflammable strata of the atmosphere, and mixing them and setting them on fire in its passage; attracted by volcanic eruptions; one thousand miles through such a medium resists less than the tenth of an inch of glass. 4. Northern lights not attracted to a point but diffused; their colours; passage of electric fire in vacuo dubious; Dr. Franklin's theory of northern lights countenanced in part by the supposition of a superior atmosphere of inflammable air; antiquity of their appearance; described in Maccabees. NOTE II ... PRIMARY COLOURS. The rainbow was in part understood before Sir Isaac Newton; the seven colours were discovered by him; Mr. Gallon's experiments on colours; manganese and lead produce colourless glass. NOTE III ... COLOURED CLOUDS. The rays refracted by the convexity of the atmosphere; the particles of air and of water are blue; shadow by means of a candle in the day; halo round the moon in a fog; bright spot in the cornea of the eye; light from cat's eyes in the dark, from a horse's eyes in a cavern, coloured by the choroid coat within the eye. NOTE IV ... COMETS. Tails of comets from rarified vapour, like northern lights, from electricity; twenty millions of miles long; expected comet. NOTE V ... SUN'S RAYS. Dispute about phlogiston; the sun the fountain from whence all phlogiston is derived; its rays not luminous till they arrive at our atmosphere; light owing to their combustion with air, whence an unknown acid; the sun is on fire only on its surface; the dark spots on it are excavations through its luminous crust. NOTE VI ... CENTRAL FIRES. Sun's heat much less than that from the fire at the earth's centre; sun's heat penetrates but a few feet in summer; some mines are warm; warm springs owing to subterraneous fire; situations of volcanos on high mountains; original nucleus of the earth; deep vallies of the ocean; distant perception of earthquakes; great attraction of mountains; variation of the compass; countenance the existence of a cavity or fluid lava within the earth. NOTE VII ... ELEMENTARY HEAT.